Recently, installing a noise-barrier wall using a material having high transparency (hereinafter simply referred to as a “transparent noise-barrier wall”) on the roadside or wayside of a road or railway by taking the right to sunlight into account is increasing in urban areas. In addition, from the viewpoint that conventional opaque materials, such as metal plate, disadvantageously impair appearance, replacement with a transparent material is proceeding. The transparent material widely used at present is a molded article of polycarbonate resin or acrylic resin out of plastic materials, because of its high strength.
This high-transparency noise-barrier wall is; however, gradually contaminated by exhaust gas from a car or a train running on the road or railroad, or by floating powder or dust in air. Therefore, a cleaning operation must be periodically performed to maintain the transparency. For maintaining the transparency of the transparent noise-barrier wall, a cleaning method by a human is most effective; however, this cleaning operation requires complete or partial closing of the road or railroad for a period of time, which is costly. Furthermore, this method has many limitations, for example, the cleaning operation cannot be performed frequently in view of traffic safety.
Titanium dioxide is widely known to have a photocatalytic function and actually, is being applied to various uses by using its antifouling, deodorizing or antimicrobial effect, and many applications thereof have been disclosed. Among these photocatalytic functions of titanium oxide, the antifouling function is used for maintaining the transparency of transparent noise-barrier walls using a polycarbonate resin or acrylic resin substrate. More specifically, a technique of allowing titanium dioxide to exist on the surface of a noise-barrier wall and to decompose the fouling component, where the surface is hydrophilized, is disclosed, for example, in JP-A-11-81250 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”), JP-A-10-37135 and JP-A-10-305091. As a result, the fouling component is easily cleaned by rainfall or the like. The titanium dioxide used in these techniques is titanium dioxide comprising anatase crystal. As such, the photocatalytic function of titanium dioxide comprising anatase crystal is widely known and is being applied to the above-described antifouling, deodorizing and antimicrobial uses, and many applications thereof have been disclosed.
This titanium dioxide comprising anatase crystal is known to have effects of photocatalytically decomposing the fouling component and hydrophilizing the surface; however, these functions are not sufficiently high. More specifically, the effect attributable to the photocatalytic reaction of the photocatalytic member applied using titanium dioxide comprising anatase crystal cannot be fully achieved in places under insufficient light irradiation (for example, in the shade such as shadow of a building) or during winter season under weak sunlight. In other words, the photocatalytic reaction effect can be utilized only in places exposed to light having sufficiently high intensity (for example, in the daytime of summer or under a lamp of artificially irradiating ultraviolet ray).